The present disclosure relates to pharmaceutically useful products obtainable from organoboronic acids. The disclosure also relates to the use of members of the aforesaid class of products, to their formulation, their preparation, their synthetic intermediates and to other subject matter.
The disclosure further relates to oral pharmaceutical formulations containing the described products.
Boronic Acid Compounds
It has been known for some years that boronic acid compounds and their derivatives, e.g. esters, have biological activities, notably as inhibitors or substrates of proteases. For example, Koehler et al. Biochemistry 10:2477, 1971 report that 2-phenylethane boronic acid inhibits the serine protease chymotrypsin at millimolar levels. The inhibition of chymotrypsin and subtilisin by arylboronic acids (phenylboronic acid, m-nitro-phenylboronic acid, m-aminophenylboronic acid, m-bromophenylboronic acid) is reported by Phillip et al, Proc. Nat. Acad. Sci. USA 68:478–480, 1971. A study of the inhibition of subtilisin Carlsberg by a variety of boronic acids, especially phenyl boronic acids substituted by Cl, Br, CH3, H2N, MeO and others, is described by Seufer-Wasserthal et al, Biorg. Med. Chem. 2(1):35–48, 1994.
In describing inhibitors or substrates of proteases, P1, P2, P3, etc. designate substrate or inhibitor residues which are amino-terminal to the scissile peptide bond, and S1, S2, S3, etc., designate the corresponding subsites of the cognate protease in accordance with: Schechter, I. and Berger, A. On the Size of the Active Site in Proteases, Biochem. Biophys. Res. Comm., 27:157–162, 1967. In thrombin, the S1 binding site or “specificity pocket” is a well defined slit in the enzyme, whilst the S2 and S3 binding subsites (also respectively called the proximal and distal hydrophobic pockets) are hydrophobic and interact strongly with, respectively, Pro and (R)-Phe, amongst others.
Pharmaceutical research into serine protease inhibitors has moved from the simple arylboronic acids to boropeptides, i.e. peptides containing a boronic acid analogue of an α-amino carboxylic acid. The boronic acid may be derivatised, often to form an ester. Shenvi (EP-A-145441 and U.S. Pat. No. 4,499,082) disclosed that peptides containing an α-aminoboronic acid with a neutral side chain were effective inhibitors of elastase and has been followed by numerous patent publications relating to boropeptide inhibitors of serine proteases. Specific, tight binding boronic add inhibitors have been reported for elastase (Ki, 0.25 nM), chymotrypsin (Ki, 0.25 nM), cathepsin G (Ki, 21 nM), α-lytic protease (Ki, 0.25 nM), dipeptidyl aminopeptidase type IV (Ki, 16 pM) and more recently thrombin (Ac-D-Phe-Pro-boroArg-OH (DuP 714 initial Ki 1.2 nM).
Claeson et al (U.S. Pat. No. 5,574,014 and others) and Kakkar et al (WO 92/07869 and family members including U.S. Pat. No. 5,648,338) disclose thrombin inhibitors having a neutral C-terminal side chain, for example an alkyl or alkoxyalkyl side chain.
Modifications of the compounds described by Kakkar et al are included in WO 96/25427, directed to peptidyl serine protease inhibitors in which the P2-P1 natural peptide linkage is replaced by another linkage. As examples of non-natural peptide linkages may be mentioned —CO2—, —CH2O—, —NHCO—, —CHYCH2—, —CH═CH—, —CO(CH2)pCO— where p is 1, 2 or 3, —COCHY—, —CO2—CH2NH—, —CHY—NX—, —N(X)CH2—N(X)CO—, —CH═C(CN)CO—, —CH(OH)—NH—, —CH(CN)—NH—, —CH(OH)—CH2— or —NH—CHOH—, where X is H or an amino protecting group and Y is H or halogen, especially F. Particular non-natural peptide linkages are —CO2— or —CH2O—.
Metternich (EP 471651 and U.S. Pat. No. 5,288,707, the latter being assigned to Trigen Limited) discloses variants of Phe-Pro-BoroArg boropeptides in which the P3 Phe is replaced by an unnatural hydrophobic amino acid such as trimethylsilylalanine, p-tert.butyl-diphenyl-silyloxymethyl-phenylalanine or p-hydroxymethylphenylalanine and the P1 side chain may be neutral (alkoxyalkyl, alkylthioalkyl or trimethylsilylalkyl).
The replacement of the P2 Pro residue of borotripeptide thrombin inhibitors by an N-substituted glycine is described in Fevig J M et al Bioorg. Med. Chem. 8: 301–306 and Rupin A et al Thromb. Haemost. 78(4):1221–1227, 1997. See also U.S. Pat. No. 5,585,360 (de Nanteuil et al).
Amparo (WO 96/20698 and family members including U.S. Pat. No. 5,698,538) discloses peptidomimetics of the structure Aryl-linker-Boro(Aa), where Boro(Aa) may be an aminoboronate residue with a non-basic side chain, for example BoroMpg. The linker is of the formula —(CH2)mCONR— (where m is 0 to 8 and R is H or certain organic groups) or analogues thereof in which the peptide linkage —CONR— is replaced by —CSNR—, —SO2NR—, —CO2—, —C(S)O— or —SO2O—. Aryl is phenyl, naphthyl or biphenyl substituted by one, two or three moieties selected from a specified group. Most typically these compounds are of the structure Aryl-(CH2)n—CONH—CHR2—BY1Y2, where R2 is for example a neutral side chain as described above and n is 0 or 1.
Non-peptide boronates have been proposed as inhibitors of proteolytic enzymes in detergent compositions. WO 92/19707 and WO 95/12655 report that arylboronates can be used as inhibitors of proteolytic enzymes in detergent compositions. WO 92/19707 discloses compounds substituted meta to the boronate group by a hydrogen bonding group, especially acetamido (—NHCOCH3), sufonamido (—NHSO2CH3) and alkylamino. WO 95/12655 teaches that ortho-substituted compounds are superior.
Boronate enzyme inhibitors have wide application, from detergents to bacterial sporulation inhibitors to pharmaceuticals. In the pharmaceutical field, there is patent literature describing boronate inhibitors of serine proteases, for example thrombin, factor Xa, kallikrein, elastase, plasmin as well as other serine proteases like prolyl endopeptidase and Ig AI Protease. Thrombin is the last protease in the coagulation pathway and acts to hydrolyse four small peptides form each molecule of fibrinogen, thus deprotecting its polymerisation sites. Once formed, the linear fibrin polymers may be cross-linked by factor XIIIa, which is itself activated by thrombin. In addition, thrombin is a potent activator of platelets, upon which it acts at specific receptors. Thrombin also potentiates its own production by the activation of factors V and VIII.
Other aminoboronate or peptidoboronate inhibitors or substrates of serine proteases are described in:    U.S. Pat. No. 4,935,493    EP 341661    WO 94/25049    WO 95/09859    WO 96/12499    WO 96/20689    Lee S-L et al, Biochemistry 36:13180–13186, 1997    Dominguez C et al, Bioorg. Med. Chem. Lett. 7:79–84, 1997    EP 471651    WO 94/20526    WO 95/20603    WO97/05161    U.S. Pat. No. 4,450,105    U.S. Pat. No. 5,106,948    U.S. Pat. No. 5,169,841.
Peptide boronic acid inhibitors of hepatic C virus protease are described in WO 01/02424.
Matteson D S Chem. Rev. 89: 1535–1551, 1989 reviews the use of α-halo boronic esters as intermediates for the synthesis of inter alia amino boronic acids and their derivatives. Matteson describes the use of pinacol boronic esters in non-chiral synthesis and the use of pinanediol boronic esters for chiral control, including in the synthesis of amino and amido boronate esters.
Contreras et al J. Organomet. Chem. 246: 213–217, 1983 describe how intramolecular N→B coordination was demonstrated by spectroscopic studies on cyclic boronic esters prepared by reacting Me2CHCMe2—BH2 with diethanolamines.
Boronic acid and ester compounds have displayed promise as inhibitors of the proteasome, a multicatalytic protease responsible for the majority of intracellular protein turnover. Ciechanover, Cell, 79:13–21, 1994, teaches that the proteasome is the proteolytic component of the ubiquitin-proteasome pathway, in which proteins are targeted for degradation by conjugation to multiple molecules of ubiquitin. Ciechanover also teaches that the ubiquitin-proteasome pathway plays a key role in a variety of important physiological processes.
Adams et al, U.S. Pat. No. 5,780,454 (1998), U.S. Pat. No. 6,066,730 (2000), U.S. Pat. No. 6,083,903 (2000) and equivalent WO 96/13266, and U.S. Pat. No. 6,297,217 (2001) describe peptide boronic ester and acid compounds useful as proteasome inhibitors. These documents also describe the use of boronic ester and acid compounds to reduce the rate of muscle protein degradation, to reduce the activity of NF-κB in a cell, to reduce the rate of degradation of p53 protein in a cell, to inhibit cyclin degradation in a cell, to inhibit the growth of a cancer cell, to inhibit antigen presentation in a cell, to inhibit NF-κB dependent cell adhesion, and to inhibit HIV replication. Brand et al, WO 98/35691, teaches that proteasome inhibitors, including boronic acid compounds, are useful for treating infarcts such as occur during stroke or myocardial infarction. Elliott et al, WO 99/15183, teaches that proteasome inhibitors are useful for treating inflammatory and autoimmune diseases.
Unfortunately, organoboronic acids can be relatively difficult to obtain in analytically pure form. Thus, alkylboronic acids and their boroxines are often air-sensitive. Korcek et al, J. Chem. Soc. Perkin Trans. 2:242, 1972, teaches that butylboronic acid is readily oxidized by air to generate 1-butanol and boric acid.
It is known that derivatisation of boronic acids as cyclic esters provides oxidation resistance. For example, Martichonok V et al J. Am. Chem. Soc. 118: 950–958, 1996 state that diethanolamine derivatisation provides protection against possible boronic acid oxidation. U.S. Pat. No. 5,681,978 (Matteson D S et al) teaches that 1,2-diols and 1,3 diols, for example pinacol, form stable cyclic boronic esters that are not easily oxidised.
Wu et al, J. Pharm. Sci., 89:758–765, 2000, discuss the stability of the compound N-(2-pyrazine) carbonyl-phenylalanine-leucine boronic acid (LDP-341, also known as bortezomib), an anti-cancer agent. It is described how “during an effort to formulate [LDP-341] for parenteral administration, the compound showed erratic stability behaviour”. The degradation pathways were investigated and it was concluded that the degradation was oxidative, the initial oxidation being attributed to peroxides or molecular oxygen and its radicals.
WO 02/059131 discloses boronic acid products which are described as stable. In particular, these products are certain boropeptides and/or boropeptidomimetics in which the boronic acid group has been derivatised with a sugar. The disclosed sugar derivatives, which have hydrophobic amino acid side chains, are of the formula
wherein:
P is hydrogen or an amino-group protecting moiety;
R is hydrogen or alkyl;
A is 0, 1 or 2;
R1, R2 and R3 are independently hydrogen, alkyl, cycloalkyl, aryl or —CH2—R5;
R5, in each instance, is one of aryl, aralkyl, alkaryl, cycloalkyl, heterocyclyl, heteroaryl, or —W—R6, where W is a chalcogen and R6 is alkyl;
where the ring portion of any of said aryl, aralkyl, alkaryl, cycloalkyl, heterocyclyl, or heteroaryl in R1, R2, R3 or R5 can be optionally substituted; and
Z1 and Z2 together form a moiety derived from a sugar, wherein the atom attached to boron in each case is an oxygen atom.
Some of the disclosed compounds are sugar derivatives of LDP-341 (see above).
Many drugs comprise an active moiety which is a carboxylic acid. There are a number of differences between carboxylic acids and boronic acids, whose effects on drug delivery, stability and transport (amongst others) have not been investigated. One feature of trivalent boron compounds is that the boron atom is sp2 hybridised, which leaves an empty 2pz orbital on the boron atom. A molecule of the type BX3 can therefore act as an electron-pair acceptor, or Lewis acid. It can use the empty 2pz orbital to pick up a pair of nonbonding electrons from a Lewis base to form a covalent bond. BF3 therefore reacts with Lewis bases such as NH3 to form acid-base complexes in which all of the atoms have a filled shell of valence electrons.
Boric acid, accordingly, can act as a Lewis acid, accepting OH−:B(OH)3+H2O→B(OH)4−+H+
Further, boronic acids of the type RB(OH)2 are dibasic and have two pKa's. Another point of distinction about boron compounds is the unusually short length of bonds to boron, for which three factors may be responsible:    1. Formation of pπ—pπ bonds;    2. Ionic-covalent resonance;    3. Reduced repulsions between non-bonding electrons.
The presumed equilibria of boronic and carboxylic acids in aqueous KOH are shown below (excluding formation of RBO22−):
Thrombosis
Hemostasis is the normal physiological condition of blood in which its components exist in dynamic equilibrium. When the equilibrium is disturbed, for instance following injury to a blood vessel, certain biochemical pathways are triggered leading, in this example, to arrest of bleeding via clot formation (coagulation). Coagulation is a dynamic and complex process in which proteolytic enzymes such as thrombin play a key role. Blood coagulation may occur through either of two cascades of zymogen activations, the extrinsic and intrinsic pathways of the coagulation cascade. Factor VIIa in the extrinsic pathway, and Factor IXa in the intrinsic pathway are important determinants of the activation of factor X to factor Xa, which itself catalyzes the activation of prothrombin to thrombin, whilst thrombin in turn catalyses the polymerization of fibrinogen monomers to fibrin polymer. The last protease in each pathway is therefore thrombin, which acts to hydrolyze four small peptides (two FpA and two FpB) from each molecule of fibrinogen, thus deprotecting its polymerization sites. Once formed, the linear fibrin polymers may be cross-linked by factor XIIIa, which is itself activated by thrombin. In addition, thrombin is a potent activator of platelets, upon which it acts at specific receptors. Thrombin activation of platelets leads to aggregation of the cells and secretion of additional factors that further accelerate the creation of a hemostatic plug. Thrombin also potentiates its own production by the activation of factors V and VIII (see Hemker and Beguin in: Jolles, et. al., “Biology and Pathology of Platelet Vessel Wall Interactions,” pp. 219–26 (1986), Crawford and Scrutton in: Bloom and Thomas, “Haemostasis and Thrombosis,” pp. 47–77, (1987), Bevers, et. al., Eur. J. Biochem. 122:429–36, 1982, Mann, Trends Biochem. Sci. 12:229–33, 1987).
Proteases are enzymes which cleave proteins at specific peptide bonds. Cuypers et al., J. Biol. Chem. 257:7086, 1982, and the references cited therein, classify proteases on a mechanistic basis into five classes: serine, cysteinyl or thiol, acid or aspartyl, threonine and metalloproteases. Members of each class catalyse the hydrolysis of peptide bonds by a similar mechanism, have similar active site amino acid residues and are susceptible to class-specific inhibitors. For example, all serine proteases that have been characterised have an active site serine residue.
The coagulation proteases thrombin, factor Xa, factor VIIa, and factor IXa are serine proteases having trypsin-like specificity for the cleavage of sequence-specific Arg-Xxx peptide bonds. As with other serine proteases, the cleavage event begins with an attack of the active site serine on the scissile bond of the substrate, resulting in the formation of a tetrahedral intermediate. This is followed by collapse of the tetrahedral intermediate to form an acyl enzyme and release of the amino terminus of the cleaved sequence. Hydrolysis of the acyl enzyme then releases the carboxy terminus.
As indicated above, platelets play two important roles in normal hemostasis. First, by aggregating, they constitute the initial hemostatic plug which immediately curtails bleeding from broken blood vessels. Secondly, the platelet surface can become activated and potentiate blood clotting, a property referred to as platelet procoagulant activity. This may be observed as an increase in the rate of activation of prothrombin by factor Xa in the presence of factor Va and Ca2+, referred to as the prothrombinase reaction. Normally, there are few (if any) clotting factors on the surface of unstimulated platelets but, when platelets are activated, negatively charged phospholipids (phosphatidylserine and phospatidylinositol) that are normally on the cytoplasmic side of the membrane become available and provide a surface on which two steps of the coagulation sequence occur. The phospholipid on the surface of activated platelets profoundly accelerates the reactions leading to the formation of thrombin, so that thrombin can be generated at a rate faster than its neutralisation by antithrombin III. The reactions that occur on the platelet surfaces are not easily inhibited by the natural anticoagulants in blood such as antithrombin III, either with or without heparin. (See Kelton and Hirsch in: Bloom and Thomas, “Haemostasis and Thrombosis,” pp. 737–760, (1981); Mustard et al in: Bloom and Thomas, “Haemostasis and Thrombosis,” pp. 503526, (1981); Goodwin et al; Biochem. J. 308:15–21, 1995).
A thrombus can be considered as an abnormal product of a normal mechanism and can be defined as a mass or deposit formed from blood constituents on a surface of the cardiovascular system, for example of the heart or a blood vessel. Thrombosis can be regarded as the pathological condition wherein improper activity of the hemostatic mechanism results in intravascular thrombus formation. Three basic types of thrombi are recognised:    the white thrombus which is usually seen in arteries and consists chiefly of platelets;    the red thrombus which is found in veins and is composed predominantly of fibrin and red cells;    the mixed thrombus which is composed of components of both white and red thrombi.
The composition of thrombi is influenced by the velocity of blood flow at their sites of formation. In general white platelet-rich thrombi form in high flow systems, while red coagulation thrombi form in regions of stasis. The high shear rate in arteries prevents the accumulation of coagulation intermediates on the arterial side of the circulation: only platelets have the capacity to form thrombi binding to the area of damage via von Willebrand factor. Such thrombi composed only of platelets are not stable and disperse. If the stimulus is strong then the thrombi will form again and then disperse continually until the stimulus has diminished. For the thrombus to stabilise, fibrin must form. In this respect, small amounts of thrombin can accumulate within the platelet thrombus and activate factor Va and stimulate the platelet procoagulant activity. These two events lead to an overall increase in the rate of activation of prothrombin by factor Xa of 300,000 fold. Fibrin deposition stabilises the platelet thrombus. Indirect thrombin inhibitors, for example heparin, are not clinically effective at inhibiting stimulation of platelet procoagulant activity. Accordingly, a therapeutic agent which inhibits platelet procoagulant activity would be useful for treating or preventing arterial thrombotic conditions.
On the venous side of circulation, the thrombus is comprised of fibrin: thrombin can accumulate because of the slower flow on the venous side and platelets play only a minor role.
Thrombosis is thus not considered to be a single indication but, rather, is a class of indications embracing distinct sub-classes for which differing therapeutic agents and/or protocols may be appropriate. Thus, regulatory authorities treat disorders such as, for example, deep vein thrombosis, cerebrovascular arterial thrombosis and pulmonary embolism as distinct indications for the purposes of licensing medicines. Two main sub-classes of thrombosis are arterial thrombosis and venous thrombosis. Arterial thrombosis includes such specific disorders as acute coronary syndromes [for example acute myocardial infarction (heart attack, caused by thrombosis in a coronary artery)], cerebrovascular arterial thrombosis (stroke, caused by thrombosis in the cerebrovascular arterial system) and peripheral arterial thrombosis. Examples of conditions caused by venous thrombosis are deep vein thrombosis and pulmonary embolism.
The management of thrombosis commonly involves the use of antiplatelet drugs (inhibitors of platelet aggregation) to control future thrombogenesis and thrombolytic agents to lyse the newly formed clot, either or both such agents being used in conjunction or combination with anticoagulants. Anticoagulants are used also preventatively (prophylactically) in the treatment of patients thought susceptible to thrombosis.
Currently, two of the most effective classes of drugs in clinical use as anticoagulants are the heparins and the vitamin K antagonists. The heparins are ill-defined mixtures of sulfated polysaccharides that bind to, and thus potentiate, the action of antithrombin III. Antithrombin III is a naturally occurring inhibitor of the activated clotting factors IXa, Xa, XIa, thrombin and probably XIIa (see Jaques, Pharmacol. Rev. 31:99–166, 1980). The vitamin K antagonists, of which warfarin is the most well-known example, act indirectly by inhibiting the post-ribosomal carboxylations of the vitamin K dependent coagulation factors II, VII, IX and X (see Hirsch, Semin. Thromb. Hemostasis 12:1–11, 1986). While effective therapies for the treatment of thrombosis, heparins and vitamin K antagonists have the unfortunate side effects of bleeding, heparin-induced thrombocytopenia (in the case of heparin) and marked interpatient variability, resulting in a small and unpredictable therapeutic safety margin.
The use of direct acting inhibitors of thrombin and other serine protease enzymes of the coagulation system is expected to alleviate these problems. To that end, a wide variety of serine protease inhibitors have been tested, including boropeptides, i.e. peptides containing a boronic acid analogue of an α-amino acid. Whilst direct acting boronic acid thrombin inhibitors have been discussed earlier in this specification, they are further described in the following section.
Neutral P1 Residue Boropeptide Thrombin Inhibitors
Claeson et al (U.S. Pat. No. 5,574,014 and others) and Kakkar et al (WO 92/07869 and family members including U.S. Pat. No. 5,648,338) disclose lipophilic thrombin inhibitors having a neutral (uncharged) C-terminal (P1) side chain, for example an alkoxyalkyl side chain.
The Claeson et al and Kakkar et al patent families disclose boronate esters containing the amino acid sequence D-Phe-Pro-BoroMpg [(R)-Phe-Pro-BoroMpg], which are highly specific inhibitors of thrombin. Of these compounds may be mentioned in particular Cbz-(R)-Phe-Pro-BoroMpg-OPinacol (also known as TRI 50b). The corresponding free boronic acid is known as TRI 50c. For further information relating to TRI 50b and related compounds, the reader is referred to the following documents:                Elgendy S et al., in The Design of Synthetic Inhibitors of Thrombin, Claeson G et al Eds, Advances in Experimental Medicine, 340:173–178, 1993.        Claeson G et al, Biochem J. 290:309–312, 1993        Tapparelli C et al, J Biol Chem, 268:4734–4741, 1993        Claeson G, in The Design of Synthetic Inhibitors of Thrombin, Claeson G et al Eds, Advances in Experimental Medicine, 340:83–91, 1993        Phillip et al, in The Design of Synthetic Inhibitors of Thrombin, Claeson G et al Eds, Advances in Experimental Medicine, 340:67–77, 1993        Tapparelli C et al, Trends Pharmacol. Sci. 14:366–376, 1993        Claeson G, Blood Coagulation and Fibrinolysis 5:411–436, 1994        Elgendy et al, Tetrahedron 50:3803–3812, 1994        Deadman J et al, J. Enzyme Inhibition 9:29–41, 1995        Deadman J et al, J. Medicinal Chemistry 38:1511–1522, 1995.        
The tripeptide sequence of TRI 50b has three chiral centres. The Phe residue is considered to be of (R)-configuration and the Pro residue of natural (S)-configuration, at least in compounds with commercially useful inhibitor activity; the Mpg residue is believed to be of (R)-configuration in isomers with commercially useful inhibitor activity. Thus, the active, or most active, TRI 50b stereoisomer is considered to be of R,S,R configuration and may be represented as:

Whilst indirect acting thrombin inhibitors have been found useful for the treatment of patients susceptible to or suffering from venous thrombosis, the same is not true of arterial thrombosis, because it would be necessary to raise the dosage used in the treatment of venous thrombosis by many times in order to treat (prevent) arterial thrombosis. Such raised dosages typically cause bleeding, which makes indirect acting thrombin inhibitors unsuitable or less preferable for treating arterial thrombosis. Heparin and its low molecular weight derivatives are indirect thrombin inhibitors, and so are unsuitable to treat arterial thrombosis. Oral direct thrombin inhibitors are in development for arterial indications but may have lower than desirable therapeutic indices, i.e. may have higher than desirable levels of bleeding at therapeutic doses.
Oral Absorption
Absorption in the gastrointestinal tract can be by an active or a passive route. Active absorption by transport mechanisms tends to be variable between individuals and with intestinal content (Gustafsson et al, Thrombosis Research, 101:171–181, 2001). The upper intestine has been identified as the principal site of oral drug absorption. In particular, the duodenum is the customary target site for absorption of orally administered drugs because of its large surface area. The intestinal mucosa acts as a barrier that controls passive transcellular absorption: the absorption of ionic species is blocked whilst the transcellular absorption of lipophilic molecules is favoured (Palm K et al., J. Pharmacol and Exp. Therapeutics, 291:435–443, 1999).
Orally administered drugs are required to be consistently and adequately absorbed. Variability of absorption between individuals or between different occasions in the same individual is unwelcome. Similarly, drugs which have a low level of bioavailability (only a small portion of the administered active agent is absorbed) are generally unacceptable.
Non-ionised compounds are favoured for passive absorption, a route associated with invariability, and are therefore preferred for consistent absorption. Lipophilic species are particularly favoured by passive absorption mechanisms and, accordingly, non-ionic, lipophilic drugs are indicated to be most favoured for consistent and high oral absorption.
Many organoboronic acid compounds may be classified as lipophilic or hydrophobic. Typically, such compounds include amongst others:    boropeptides of which all or a majority of the amino acids are hydrophobic    boropeptides of which at least half of the amino acids are hydrophobic and which have a hydrophobic N-terminal substituent (amino protecting group)    non-peptides based on hydrophobic moieties.
Typical functionalities required for interaction of drugs with their physiological targets are functional groups such as carboxylic and sulphonic acids. These groups exist as the protonated form in the stomach (at pH 2–3), but will be ionised to some extent at the higher pH of the intestinal fluid. One strategy that has been used to avoid the ionisation of the carboxylates or sulphonates is to present them as ester forms, which are cleaved once absorbed into the vascular lumen.
For example, the direct acting thrombin inhibitor melagatran, which has sub-optimal gastrointestinal absorption, has terminal carboxy and amidino groups and is a pure zwitterion at pH 8–10 when the carboxylic acid and amidino groups are both charged. A prodrug H 376/95 was therefore developed which has protecting groups for the carboxylic acid and for the amidine and is a more lipophilic molecule than melagatran. The prodrug has a permeability coefficient across cultured epithelial Caco-2 cells 80 times higher than that of melagatran and oral bioavailability 2.7–5.5 times higher than that of melagatran as well as much smaller variability in the area under the drug plasma concentration vs. time curve (Gustafsson et al, Thrombosis Research, 101:171–181, 2001).
Boronic acids are divalent functional groups, with boron-oxygen bond lengths (1.6 Å) more typical of single bonds, unlike superficially comparable C—O and S—O bonds in carboxylic and sulphonic acids. Consequently the boronic acid group has two ionisation potentials. The boronic acid group will be partly ionised at pH's of the duodenal fluid and not suited to the desired passive duodenal uptake. Thus, a charged boronate inhibitor H-D-PheProBoroArg is absorbed by a predominantly active transport mechanism (Saitoh, H. and Aungst, B. J., Pharm. Res., 16:1786–1789, 1999).
Oral Absorption of Boropeptides, Boropeptidomimetics and other Organoboronates
The boronate ester group of TRI 50b is rapidly cleaved in the conditions of the plasma to form the corresponding boronic acid group, which is considered to be the active moiety which inhibits the catalytic site of thrombin.
The peptide boronic acid formed by such cleavage of TRI 50b (the acid is designated TRI 50c) is relatively insoluble in water, especially at acidic or neutral pH, and tends to be poorly absorbed in the stomach and duodenum. The acid has the structure Cbz-Phe-Pro-BoroMpg-OH.
Whereas the peptide boronic acid Cbz-Phe-Pro-BoroMpg-OH is partly ionised under duodenal conditions and, to that extent, unfavoured for passive transport, esters of the acid are designed for a high rate of passive (thus consistent) transport. The tripeptide sequence Phe-Pro-Mpg has a non-basic P1 side chain (specifically, methoxypropyl), such that the tripeptide consists of three non-polar amino acids. The esters of the peptide boronic acid are non-ionisable and the ester-forming species further impart lipophilic properties, so encouraging a high rate of passive transport.
Computational techniques have confirmed that TRI 50b and other diol esters of Cbz-Phe-Pro-BoroMpg-OH can be predicted to have good bioavailability. Thus, polar surface area (PSAd) is a parameter predictive of bioavailability and PSAd values of greater than 60 Å correlate well with passive transcellular transport and with bioavailability of known drugs (Kelder, J. Pharm. Res., 16:1514–1519, 1999). Measurements for diol esters of the above peptide boronic acid, including the pinacol ester TRI 50b, show that the diol esters have PSAd values well above 60 Å, predictive of passive transport and good bioavailability as shown in Table 1:
TABLE 1PSAd values of selected diol esters f Cbz-Phe-Pro-BoroMpg-OHDiolPSAd ValuePinacol98.74Pinanediol90.64
The corresponding monohydroxy alcohol (e.g. alkanol) esters were considered too unstable, spontaneously cleaving to liberate the acid in-vitro. Esters of diols such as pinanediol and pinacol have enhanced kinetic stability over esters of monohydroxy alcohols, in that after partial hydrolysis to the mono-ester derivative they will tend to reassociate by a facile intra-molecular reaction.